The invention relates to a data carrier, notably a chip card.
U.S. Pat. No. 5,406,064 discloses an IC card which can operate at option in an operating mode or in an energy-saving mode. The operations in the operating mode are executed only in conformity with a command signal which is supplied by a predetermined, external device, and in the energy-saving mode the associated operations can be terminated after the reception of a predetermined enable signal, thus enabling a return to the operating mode. The IC card includes an information processing circuit and an enable signal generating circuit for delivering the enable signal to the information processing circuit at the instant at which the command signal is received from the external device. The information processing circuit executes the process corresponding to the command signal received in the operating mode and subsequently triggers the IC card to switch over to the energy-saving mode. A portable terminal, enabling the operation of the IC card in the energy-saving mode, includes a clock signal generating circuit which is arranged to generate a clock signal of a frequency which is lower than the nominal value of the clock frequency of the IC card. An interface is arranged for the transfer of data to the IC card, the data transfer rate being proportional to a ratio of the low frequency of the clock signal from the clock signal generating circuit to the nominal value of the clock frequency of the IC card.
It has been found that such an arrangement does not enable optimum use of the energy applied to the IC card.
It is an object of the invention to construct a data carrier in such a manner that optimum use can be made of the energy applied to such a data carrier.
This object is achieved according to the invention by means of a data carrier, notably a chip card, which includes a data processing unit and at least one contactless interface via which the data processing unit can be coupled to a read/write apparatus in order to exchange data signals and to take up electrical energy for the operation of the data processing unit, the data processing unit being constructed at least mainly of at least substantially asynchronously operating logic components (asynchronous logic).
Generally speaking, a simpler construction and simpler operation of a data processing unit on a data carrier are achieved in that all or substantially all logic components of the data processing unit are controlled in synchronism by one clock signal. As is known, the execution in time of operations for the processing of data signals can thus be very accurately predetermined.
In the data carrier according to the invention, however, the data processing unit is constructed in such a manner that at least the majority of the logic components therein, if not all components, are arranged as asynchronously operating logic components. Such asynchronously operating logic components are also referred to as asynchronous logic. They are distinct from synchronously operating logic components in that they are no longer controlled by a preferably centrally generated clock signal, but operate rather like under xe2x80x9chandshakexe2x80x9d control. Such logic components are activated only in response to a request signal. Once they have executed their operation and their result is available, they signal so by supplying a xe2x80x9cfinishedxe2x80x9d message. The co-operation of asynchronously operating logic components takes place by the propagation of, for example the xe2x80x9cfinishedxe2x80x9d message from a first logic component as a request signal to the next logic component in the series of operating steps to be performed for the data signals. A predetermined time frame, like in the case of a predetermined clock signal, no longer occurs; the period of time required for the relevant processing of data signal results exclusively from the combination of the processing times in the individual logic components to be successively traversed. It has been found that the processing times in the logic components are strongly dependent on the supply voltage applied thereto. If a fixed clock signal were given, a decreasing supply voltage could give rise to the problem that errors occur in the processing of the data signals due to processing times which exceed the predetermined clock periods. Using asynchronous logic, however, the processing time for the data signals increases; consequently, errors cannot occur.
This is advantageous notably when in the present case the data carrier receives only a very small amount of electrical energy via the contactless interface. The overall period of time required for the processing of the data signals then becomes longer and longer, but the exceeding of a predetermined clock period cannot cause errors in the processing of the data signals. Because of the automatic adaptation of the processing times of the data signal processing in the asynchronously operating logic components, the energy required by the data processing unit is always exactly adapted to the electrical energy supplied via the contactless interface. Consequently, the data processing unit will not be forced into a mode of operation in which its instantaneous energy consumption exceeds, be it perhaps only briefly, the highest energy that can be supplied via the contactless interface. Conversely, the construction of the data carrier according to the invention offers the possibility of completing individual signal processing steps, or a set of signal processing steps, within a given period of time and with a minimum energy consumption. Should the supplied electrical energy decrease, the signal processing in the asynchronously operating logic components automatically slows down and, conversely, it can be deliberately adjusted, by reduction of the energy supply, to a predetermined value which is accompanied by a saving of energy.
This aspect can be used to particularly good advantage when two parts of the data processing unit have to execute a different number of signal processing steps within a given period of time. For the smaller number of signal processing steps a lower signal processing speed can then be selected, so that in this part of the data processing unit electrical energy can be saved in comparison with the other part of the data processing unit.
However, the construction of the data processing unit using asynchronous logic also offers the possibility of quasi-synchronous operation via central timing of the request signals, for example by means of a synchronization clock. To this end, synchronization of the command execution is imposed at event limits which are provided, for example by a timer and correspond at least approximately to command limits generated in the case of synchronously operating logic components, so that outside the command limits the data processing unit behaves as if it were operating synchronously. This mode of operation is particularly useful for fault finding in the programs or program sections to be executed by the data processing unit. It can be deactivated for the intended operation of the data carrier, for example by means of a switch.
The fully asynchronous mode of operation of the data processing unit according to the invention imposes different execution times of unpredictable length for the individual data signals to be processed. Consequently, unauthorized access to the data carrier, aimed at finding out the execution times for the individual signal processing steps performed on the data signals, is inhibited in practice.
The fully asynchronous mode of operation of the data carrier according to the invention, moreover, inhibits unauthorized accessing of data signals via differential power analysis. Such a method of attack utilizes signal waveforms which arise at the contactless interface due to the operation of the data processing unit. It aims to extract information as regards the processed data signals by correlating different signal patterns produced for different data signals. Because a strictly synchronous, clock-controlled execution of operations is required for such a method, the use of asynchronous logic in a mode of operation which is not synchronized by a clock signal prevents such an attack from becoming successful.
The contactless interface and the data processing unit in a preferred embodiment of the data carrier according to the invention are coupled via an asynchronous transmission/receiving circuit which is included in the data processing unit. Such asynchronous transmission/receiving circuits are generally known as UART. They enable the reception of a data signal of a predetermined clock frequency from the read/write apparatus and the propagation of this data signal, asynchronously with respect to the reception, to the data processing unit and, conversely, asynchronous reception of such a data signal from the data processing unit and its synchronous propagation to the read/write apparatus.
Preferably, in the data carrier according to the invention time interleaved operation takes place of individual stages within at least the data processing unit. Such time interleaved operation can also advantageously take place within the contactless interface. It is thus achieved that only few stages consuming electrical energy are in operation at a time within the data carrier, so that an as uniform as possible, low energy consumption of the data carrier can be achieved via the contactless interface, without inadmissibly high power peaks occurring. This benefits the transfer characteristic of the contactless interface for the electrical energy.
The contactless interface for the electrical energy supply for the operation of the data processing unit in a further embodiment of the data carrier according to the invention has the function of an at least substantially ideal current source. This means that the contactless interface delivers an at least essentially constant current in order to supply the data processing unit with electrical energy, said current being, at least in wide ranges, at least substantially independent of the electrical voltage on a terminal via which said current is applied from the contactless interface to the data processing unit. As a result of this construction, such a control function is realized for the data carrier according to the invention that, in the case of a high energy consumption by the data processing unit, said supply voltage, with which the current is supplied, automatically decreases. As the supply voltage decreases, however, the processing speed for the data signals in the data processing unit also decreases. This means that as the supply voltage is lower, the processing times become longer and hence also the signal delay which is imposed on the data signals in the data processing unit due to these processing times. Thus, as the data processing unit becomes slower, its activity decreases, i.e. the number of data processing operations per unit of time decreases. However, its energy consumption then also decreases, with the result that the current consumption decreases, thus enabling an increase of the supply voltage. Self-control of the performance of the data processing unit is thus very simply and effectively realized in conformity with the supply of electrical energy via the contactless interface.